Ships for transporting liquefied natural gas have a plurality of tanks in which to store the cargo. The liquefied natural gas is stored in these tanks, at atmospheric pressure, at around −162° C., and is thus in a state of diphasic liquid-vapor equilibrium such that the heat flux applied through the walls of the tanks tends to cause the liquefied natural gas to evaporate.
In order to avoid the generation of overpressures inside the tanks, each tank is associated with a sealed pipe for removing the vapor produced by the evaporation of the liquefied natural gas. Such a sealed vapor removal pipe is notably described in application WO2013093261, for example. The pipe passes through a wall of the tank and opens in an upper part of the internal space of the tank and thus defines a vapor passage between the interior space of the tank and a vapor collector arranged outside the tank. The vapor thus collected may then be transmitted to a re-liquefaction apparatus so that the fluid can then be reintroduced into the tank, to energy production equipment or to a flare stack provided on the deck of the ship.
However, under certain incident conditions, when the tank fill level is at a maximum and the ship has run aground in a position in which it has an inclination in that it is listing and/or an inclination in terms of trim to (a) significant extent(s), there is a risk that the vapor removal pipe will open into the liquid phase and therefore no longer be in contact with the vapor phase stored in the tank. In such circumstances isolated pockets of gas in the vapor phase are liable to form inside the tanks. Now, such pockets of gas are liable to give rise to overpressures which may damage the tanks and/or lead to liquid phase being expelled to the outside of the tank through the aforementioned vapor removal pipe.
In order to limit the probabilities of such isolated pockets of gas forming, it is admittedly known practice to limit the maximum tank fill level. However, such limiting of the tank fill level leads to a loss of revenue for the carriers and is therefore not entirely satisfactory.